Turbocharged engine systems include engines having combustion chambers for combusting fuel and oxidants for conversion into mechanical power, and also having induction subsystems to convey induction gases to the combustion chambers. Such engine systems also have exhaust subsystems to carry exhaust gases away from the combustion chambers, muffle engine exhaust noise, and reduce exhaust gas particulates and oxides of nitrogen (NOx), which tend to increase as engine combustion temperatures increase. Exhaust gas is often recirculated out of the exhaust gas subsystem, into the induction subsystem for mixture with fresh air, and back to the engine. Exhaust gas recirculation increases the amount of inert gas and concomitantly reduces the amount of oxygen in the induction gases, thereby reducing engine combustion temperatures and, thus, reducing NOx formation.
Engine knock may include sound and other effects associated with non-uniform combustion of an oxidant/fuel mixture. For example, in a spark-ignition engine, a spark plug ignites a first portion of an oxidant/fuel mixture, thereby creating a flame front that combusts the first portion and travels away from the spark plug toward unburned portions of the mixture. Hot combustion gases behind the flame front rapidly compress the unburned portions of the mixture ahead of the flame front, thereby quickly raising the temperature of the unburned portions above a self-ignition temperature of the mixture. If the flame front travels fast enough to consume the unburned portions before the unburned portions self-ignite, then normal combustion without knock takes place. Otherwise, the unburned portions nearly instantly combust, thereby producing a strong shock wave in the combustion chamber that causes the characteristic metallic pinging or knocking sound.